In recent years, the quality regulation values of gas oil have tended to become severe worldwide in order to improve air environment. Particularly, sulfur compounds in gas oil may affect durability of post-treatment facilities which are expected as countermeasures against exhaust gas from diesel vehicles, such as an oxidation catalyst, a nitrogen oxide (NOx) reducing catalyst, and a continuous regenerative filter for removing diesel exhaust particles. A sulfur reduction in gas oil is thus desired. It is expected that the greater the sulfur reduction in gas oil, the greater the effect of suppressing production of sulfate in exhaust gas, the greater the effect of suppressing deterioration of a nitrogen-oxide reducing catalyst, and the greater the effect of reducing production of particulate matter on a post-treatment catalyst, whereby emission of nitrogen oxides and particulate matter can be reduced.
Under such a situation, efforts to develop an ultra-deep desulfurization technology which can significantly reduce sulfur content in gas oil have been undertaken. As the technology for reducing the sulfur content in gas oil, a method of employing hydrodesulfirization operating conditions that promote desulfurization, for example, increasing the reaction temperature, decreasing the liquid hourly space velocity (LHSV) and the like can be considered. However, if the reaction temperature is increased, carbonaceous materials deposit on the catalyst and the activity of the catalyst quickly decreases. On the other hand, if the LHSV is decreased, the desulfurization capability is improved. However, since the throughput is decreased, it is necessary to expand the scale of equipment to compensate for the decreased throughput. In addition, severer operating conditions excessively hydrogenate aromatics and increase hydrogen consumption, resulting in a high production cost.
A method of subjecting a hydrodesulfurized oil with a sulfur content of 5 to 10 mass ppm to deep hydrodesulfurization in the presence of a hydrodesulfurization catalyst to produce a deep-hydrodesulfurized gas oil with a sulfur content of not more than 5 mass ppm, and processing the resulting deep-hydrodesulfurized gas oil together with an unhydrodesulfurized oil and/or a hydrodesulfurized oil in the presence of a hydrodesulfurizing catalyst to obtain gas oil composition having a sulfur content of not more than 5 mass ppm and a total aromatic content of 3 to 12 vol % or not more than 10 vol % has been disclosed (Patent document 1 and 2). However, the deep-hydrodesulfurization treatment must be carried out under a high hydrogen pressure of 2 to 10 MPa, requires a large reactor due to the need of a low LHSV of 0.1 to 2 hr−1, and involves significantly high hydrogen consumption since almost all the aromatics are hydrogenated in the reaction. Therefore the method has problem in the viewpoint of economical production. Moreover, the resulting gas oil has a low density because of reducing the total aromatic content to a level of 12 vol % or less, which leads to poor fuel consumption (mileage) and low output power due to the low calorific value of the gas oil. In addition, there is concern that the fuel is leaked by adverse affect to a seal rubber material or the like used in the fuel injection system.
The applicant of this invention has previously proposed a method for reducing the sulfur content in gas oil to 10 mass ppm or less with almost no reduction of aromatic content by adsorption desulfurization (Patent document 3). However, the adsorption desulfurization has only low capability of adsorbing sulfur and requires frequent regeneration of the adsorbent when operating over a long period of time. The method is thus uneconomical.    [Patent document 1] JP-A-2004-269683    [Patent document 2] JP-A-2004-269685    [Patent document 3] WO 03/097771